High-strength galvanized steel sheet and method for manufacturing same

Abstract

A high-strength galvanized steel sheet includes a steel sheet having a steel composition having a specific component composition, a steel structure containing martensite and bainite at more than or equal to 70% (including 100%), ferrite at less than 20% (including 0%), and retained austenite at less than 5% (including 0%) in terms of area ratio, the amount of diffusible hydrogen in steel being less than or equal to 0.20 mass ppm; and a galvanizing layer provided on a surface of the steel sheet, having a content amount of Fe of 8 to 15% in mass %, and having an coating weight per one surface of 20 to 120 g/m2, wherein the amount of Mn oxides contained in the galvanizing layer is less than or equal to 0.050 g/m2, and a tensile strength is more than or equal to 1100 MPa and a yield ratio is more than or equal to 0.85.

The invention claimed is: 
     
       1. A high-strength galvanized steel sheet comprising:
 a steel sheet having a steel composition having 
 a component composition containing, in mass %, 
 C: more than or equal to 0.10% and less than or equal to 0.30%, 
 Si: less than 1.2%, 
 Mn: more than or equal to 2.0% and less than or equal to 3.5%, 
 P: less than or equal to 0.010%, 
 S: less than or equal to 0.002%, 
 Al: less than or equal to 1%, 
 N: less than or equal to 0.006%, and 
 the balance including Fe and unavoidable impurities, and 
 a steel structure containing martensite and bainite at more than or equal to 70% (including 100%), ferrite at less than 20% (including 0%), and retained austenite at less than 5% (including 0%) in terms of area ratio, 
 an amount of diffusible hydrogen in steel being less than or equal to 0.20 mass ppm of molecular hydrogen; and a galvanizing layer provided on a surface of the steel sheet, having a content amount of Fe of 8 to 15% in mass %, and having an coating weight per one surface of 20 to 120 g/m 2 , 
 wherein an amount of Mn oxides contained in the galvanizing layer is less than or equal to 0.050 g/m 2 , and 
 a tensile strength is more than or equal to 1100 MPa and a yield ratio is more than or equal to 0.85. 
 
     
     
       2. The high-strength galvanized steel sheet according to  claim 1 , wherein the component composition further contains at least one selected from the groups of A to C, in mass %,
 group A:
 at least one selected from, one or more of Ti, Nb, V, and Zr: 0.005 to 0.1% in total, 
 one or more of Mo, Cr, Cu, and Ni: 0.005 to 0.5% in total, and 
 B: 0.0003 to 0.005%; 
 
 group B:
 any one or two selected from Sb: 0.001 to 0.1% and Sn: 0.001 to 0.1%; and 
 
 group C:
 Ca: less than or equal to 0.0010%. 
 
 
     
     
       3. A method for manufacturing the high-strength galvanized steel sheet according to  claim 1 , comprising:
 an annealing step of allowing a cold rolled material having the component composition according to  claim 1  to stay in an atmosphere within an annealing furnace with a hydrogen concentration H of more than or equal to 1 vol % and less than or equal to 13 vol %, in a temperature region of temperature within the annealing furnace T of an A c3  point−20° C. to 900° C. for more than or equal to 5 seconds, then performing cooling, and allowing the cold rolled material to stay in a temperature region of 400 to 550° C. for more than or equal to 10 seconds; 
 a coating step of subjecting a steel sheet after the annealing step to coating treatment and alloying treatment, and performing cooling down to less than or equal to 100° C. at an average cooling rate of more than or equal to 3° C./s; and 
 a post heat treatment step of allowing a coated steel sheet after the coating step to stay in an atmosphere within a furnace with a hydrogen concentration H of less than or equal to 10 vol % and a dew point Dp of less than or equal to 50° C., at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. for more than or equal to a time t (hr) that is more than or equal to 0.005 (hr) and satisfies a formula (1)
   130−18.3× In ( t )≤ T   (1).
 
 
 
     
     
       4. The method for manufacturing a high-strength galvanized steel sheet according to  claim 3 , wherein, after the coating step, temper rolling is performed at an extension rate of more than or equal to 0.1%. 
     
     
       5. The method for manufacturing a high-strength galvanized steel sheet according to  claim 4 , wherein width trimming is performed after the post heat treatment step. 
     
     
       6. The method for manufacturing a high-strength galvanized steel sheet according to  claim 4 ,
 wherein width trimming is performed before the post heat treatment step, and 
 a staying time t (hr) for staying at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. in the post heat treatment step is more than or equal to 0.005 (hr) and satisfies a formula (2):
   115−18.3× In ( t )≤ T   (2).
 
 
 
     
     
       7. A method for manufacturing a high-strength galvanized steel sheet according to  claim 1 , comprising:
 a pretreatment step of heating a cold rolled material, having the component composition according to  claim 1 , up to an A c3  point to the A c3  point+50° C. in a temperature region and performing pickling after said heating to form a pickled sheet; 
 an annealing step of allowing the pickled sheet to stay in an atmosphere within an annealing furnace with a hydrogen concentration H of more than or equal to 1 vol % and less than or equal to 13 vol %, in a temperature region of temperature within an annealing furnaces T of an A c3  point−20° C. to 900° C. for more than or equal to 5 seconds, then performing cooling, and allowing the cold rolled material to stay in a temperature region of 400 to 550° C. for more than or equal to 10 seconds; 
 a coating step of subjecting a steel sheet after the annealing step to coating treatment and alloying treatment, and performing cooling down to less than or equal to 100° C. at an average cooling rate of more than or equal to 3° C./s; and 
 a post heat treatment step of allowing a coated steel sheet after the coating step to stay in an atmosphere within a furnace with a hydrogen concentration H of less than or equal to 10 vol % and a dew point Dp of less than or equal to 50° C., at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. for more than or equal to a time t (hr) that is more than or equal to 0.005 (hr) and satisfies a formula (1):
   130−18.3× In ( t )≤ T   (1).
 
 
 
     
     
       8. The method for manufacturing a high-strength galvanized steel sheet according to  claim 7 , wherein, after the coating step, temper rolling is performed at an extension rate of more than or equal to 0.1%. 
     
     
       9. The method for manufacturing a high-strength galvanized steel sheet according to  claim 8 , wherein width trimming is performed after the post heat treatment step. 
     
     
       10. The method for manufacturing a high-strength galvanized steel sheet according to  claim 8 ,
 wherein width trimming is performed before the post heat treatment step, and 
 a staying time t (hr) for staying at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. in the post heat treatment step is more than or equal to 0.005 (hr) and satisfies a formula (2):
   115−18.3× In ( t )≤ T   (2).
 
 
 
     
     
       11. A method for manufacturing the high-strength galvanized steel sheet according to  claim 2 , comprising:
 an annealing step of allowing a cold rolled material having the component composition according to  claim 2  to stay in an atmosphere within an annealing furnace with a hydrogen concentration H of more than or equal to 1 vol % and less than or equal to 13 vol %, in a temperature region of temperature within the annealing furnace T of an A c3  point−20° C. to 900° C. for more than or equal to 5 seconds, then performing cooling, and allowing the cold rolled material to stay in a temperature region of 400 to 550° C. for more than or equal to 10 seconds; 
 a coating step of subjecting a steel sheet after the annealing step to coating treatment and alloying treatment, and performing cooling down to less than or equal to 100° C. at an average cooling rate of more than or equal to 3° C./s; and 
 a post heat treatment step of allowing a coated steel sheet after the coating step to stay in an atmosphere within a furnace with a hydrogen concentration H of less than or equal to 10 vol % and a dew point Dp of less than or equal to 50° C., at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. for more than or equal to a time t (hr) that is more than or equal to 0.005 (hr) and satisfies a formula (1)
   130−18.3× In ( t )≤ T   (1).
 
 
 
     
     
       12. The method for manufacturing a high-strength galvanized steel sheet according to  claim 11 , wherein, after the coating step, temper rolling is performed at an extension rate of more than or equal to 0.1%. 
     
     
       13. The method for manufacturing a high-strength galvanized steel sheet according to  claim 12 , wherein width trimming is performed after the post heat treatment step. 
     
     
       14. The method for manufacturing a high-strength galvanized steel sheet according to  claim 12 ,
 wherein width trimming is performed before the post heat treatment step, and 
 a staying time t (hr) for staying at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. in the post heat treatment step is more than or equal to 0.005 (hr) and satisfies a formula (2):
   115−18.3× In ( t )≤ T   (2).
 
 
 
     
     
       15. A method for manufacturing a high-strength galvanized steel sheet according to  claim 2 , comprising:
 a pretreatment step of heating a cold rolled material, having the component composition according to  claim 2 , up to an A c3  point to the A c3  point+50° C. in a temperature region and performing pickling after said heating to form a pickled sheet; 
 an annealing step of allowing the pickled sheet to stay in an atmosphere within an annealing furnace with a hydrogen concentration H of more than or equal to 1 vol % and less than or equal to 13 vol %, in a temperature region of temperature within an annealing furnaces T of an A c3  point−20° C. to 900° C. for more than or equal to 5 seconds, then performing cooling, and allowing the cold rolled material to stay in a temperature region of 400 to 550° C. for more than or equal to 10 seconds; 
 a coating step of subjecting a steel sheet after the annealing step to coating treatment and alloying treatment, and performing cooling down to less than or equal to 100° C. at an average cooling rate of more than or equal to 3° C./s; and 
 a post heat treatment step of allowing a coated steel sheet after the coating step to stay in an atmosphere within a furnace with a hydrogen concentration H of less than or equal to 10 vol % and a dew point Dp of less than or equal to 50° C., at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. for more than or equal to a time t (hr) that is more than or equal to 0.005 (hr) and satisfies a formula (1):
   130−18.3× In ( t )≤ T   (1).
 
 
 
     
     
       16. The method for manufacturing a high-strength galvanized steel sheet according to  claim 15 , wherein, after the coating step, temper rolling is performed at an extension rate of more than or equal to 0.1%. 
     
     
       17. The method for manufacturing a high-strength galvanized steel sheet according to  claim 16 , wherein width trimming is performed after the post heat treatment step. 
     
     
       18. The method for manufacturing a high-strength galvanized steel sheet according to  claim 16 ,
 wherein width trimming is performed before the post heat treatment step, and 
 a staying time t (hr) for staying at a temperature T (° C.) of more than or equal to 200° C. and less than or equal to 350° C. in the post heat treatment step is more than or equal to 0.005 (hr) and satisfies a formula (2):
   115−18.3× In ( t )≤ T   (2).